In recent years, a digital watermark technology for embedding and extracting information in and from content such as image or video is actively developed to achieve a purpose such as copyright protection, tracing, forgery prevention, tamper detection, or addition of meta-information for image retrieval. The digital watermark technology enables embedding of information in an image, so that a detectability of the information embedded in the image (hereinafter, referred to as “embedded information”) can be decreased, resulting in protection of the embedded information. On the other hand, a technique of extracting the embedded information with high accuracy is a key for success of an information embedding technology including the digital watermark technology.
Conventionally, a technology for increasing the detectability of the embedded information by assuring robustness against the decreased detectability of the embedded information has been developed. For example, Japanese Patent Application Laid-open No. 2000-216983, Japanese Patent Application Laid-open No. 2001-333267, and Japanese Patent Application Laid-open No. 2006-135922 disclose a technology for redundantly embedding information. Furthermore, a technology for applying embedded information as an error correction code is disclosed in, for example, Japanese Patent Application Laid-open No. 2002-010058, Japanese Patent Application Laid-open No. 2000-187441, and Japanese Patent Application Laid-open No. 2004-221715.
In the conventional technologies disclosed above, embedded information is embedded in an image through redundant recording thereof so that the detectability of digital watermark can be more decreased, resulting in increasing security of the digital watermark. However, because the embedded information is redundantly recorded, when the embedded information is to be extracted from the image, larger number of pieces of information than desired pieces of information to be extracted need to be detected to decode smaller amount of embedded information than redundantly-recorded information. Thus, there is a problem that, in processing of both embedding and extracting information, image processing cannot be performed efficiently in terms of calculation amounts.
The present invention has been made to solve the above problems in the conventional technology and it is an object of the present invention to increase a processing speed when information is extracted from a content in which embedded information is redundantly embedded.